This invention relates to a process of manufacture of polyvinylbutyral (PVB) sheeting having a desired uniform adhesivity for use in laminated safety glass. More particularly, it relates to a process for monitoring the properties and controlling the uniformity of PVB sheeting, in particular the acidity of the sheet and adding water to control the resultant adhesivity of the sheet to glass.
The most widely used vehicle windshield is a three-ply laminate comprising an energy absorbing plastic sheet sandwiched between two glass sheets. Such windshields are capable of absorbing a blow from the head of an occupant with resistance to penetration and shattering. In so doing, the windshields dissipate a substantial amount of impact energy as they undergo plastic deformation.
Polyvinylbutyral resins are capable of being cast or preferably extruded by processes analogous to those used to compound and extrude thermoplastics. Although it is possible to form a plastic sheet directly from polyvinylbutyral resins, the glass transition temperature of approximately 70.degree. C. for PVB resin is too high for laminated glass application. Plasticizer is mixed with PVB resin to lower the glass transition temperature to near 25.degree. C. In the process to form the sheeting film from its molten state, temperatures must be high enough to obtain processable film forming melt viscosities and intimate mixing of the PVB resin and plasticizer.
Plasticizer is typically a large fraction of the final sheet composition. In PVB sheeting, for example, plasticizers contribute 15% to 50% of the film weight, typically 25% to 30%. Choice of plasticizer is limited to materials that are low in volatility, chemically stable under processing conditions, and compatible with the particular acetal resin selected. Diester plasticizers having from 16 to 28 carbon atoms are preferred, and plasticizers based on difunctional acids as well as glycols are also used. Commercially available plasticizers and their compatibility with polyvinylbutyral resin are listed in Modern Plastics Encyclopedia 1994-95, Vol. 71, No. 12, pp. C-99 to C-109. Examples of suitable plasticizers include esters of aliphatic diols with aliphatic carboxylic acids, in particular the esters of di-, tri- and tetraethylene glycol with aliphatic carboxylic acids having 6 to 10 carbon atoms, e.g., triethylene glycol di-(2-ethyl-butyrate), triethylene glycol di-n-heptanoate, or tetraethylene glycol di-n-heptanoate. Also suitable are esters or mixed esters of dicarboxylic acids, preferably adipic, sebasic or phthalic acid with aliphatic alcohols or mixed alcohols. For example, di-n-hexyl adipate, 2-ethylhexyl cyclohexyl adipate, dibutyl phthalate, butyl benzyl phthalate, or hexylphthalate. Also, polymeric propylene oxide oligomers, as disclosed in U.S. Pat. No. 4,335,036, can be used as polyvinylbutyral plasticizers.
Plasticizers are hydrolyzable to various degrees during manufacturing processes for PVB sheeting. Ester-based plasticizers react with water to break down into alcohols and acids. The adhesivity of PVB sheeting is directly proportional to its acidity. In order to be suitable for use in glazing applications, including automobile windshields and architectural glass, PVB sheeting requires controlled, uniform adhesivity. Heretofore, the acidity has been controlled by the use of additives of the type described in U.S. Pat. No. 4,292,372.